1. Field of the Invention
The present invention relates to an ophthalmic ultrasonic diagnosing apparatus for measuring an ocular axial length of an examinee's eye or the sizes of constituent parts of the eye.
2. Description of Related Art
Conventionally, there is known an ophthalmic ultrasonic diagnosing apparatus for measuring an ocular axial length that is arranged to obtain the positions of constituent parts of an eyeball by transmitting an ultrasonic wave from a transducer included in an ultrasonic probe to the constituent parts, and receiving to process return echoes therefrom, and measure the ocular axial length (see Japanese Patent Publication Laid-Open No 2001-187022). It is to be noted that the ocular axial length is basic data to be used for prescribing an intraocular lens (hereinafter, referred to as an IOL), which is to be implanted in an eye after an opaque crystalline lens of a cataract is removed therefrom.
Specifically, in the measurement of the ocular axial length of a phakic eye, the apparatus identifies a return echo 102 that exceeds a detection level 106a first from the anterior-segment side of the eye as the return echo from the cornea, a return echo 103a that exceeds the detection level 106a second as the return echo from the front surface of the crystalline lens, and a return echo 103b that exceeds the detection level 106a third as the return echo from the rear surface of the crystalline lens, as shown in FIG. 4. Then, the apparatus identifies a return echo 104 that exceeds a detection level 106b first behind a position P1 located a predetermined distance W apart from the position of the cornea (the predetermined distance W is 12 mm, for example, which is established such that the return echoes within the predetermined distance W are distinguished from a return echo from the retina) as the return echo from the retina. Based on a distance between a rising position 107 of the corneal return echo 102 and a rising position 109 of the retinal return echo 104, the apparatus automatically obtains the ocular axial length.
If the return echoes from the constituent parts of the anterior segment and the retina are difficult to be identified automatically, manual measurement may be carried out such that an examiner observes waveforms of the return echoes (A-mode waveforms) displayed on a display and manually establishes gates to identify the positions of the constituent parts based on the examiner's experience (see Japanese Patent Publication Laid-Open No. 2008-29468).
In the measurement of the ocular axial length of an eye with previously implanted IOL (hereinafter, referred to as an IOL implanted eye), which is a case where a new IOL is prescribed to an examinee having an IOL implanted eye, the return echo 103b from the rear surface of the crystalline lens as shown in FIG. 4 does not appear but instead multiple return echoes could remarkably appear subsequent to a return echo from the IOL because an ultrasonic signal repeatedly returns between the IOL and the tip (the transducer) of the ultrasonic probe (or the cornea). The appearance of the multiple return echoes result from a great difference between physical properties of the eye constituent parts and the IOL.
However, the use of the above-described criteria of the identification of the retinal position for the IOL implanted eye leads to false detection of the multiple return echoes as the retinal return echo, and a wrong measurement result is output. If the examiner is not capable of recognizing occurrence of the multiple return echoes, the examiner cannot notice the fact that the ocular axial length is wrongly measured, and accordingly cannot prescribe an IOL with correct power. The function of the manual measurement such that the examiner manually establishes the gates is sometimes used according to the circumstances; however, it remains difficult to distinguish the multiple return echoes from the retinal return echo.